The First World War is often referred to as "a 20th century war fought with 19th century tactics". It was the first "world" war in the true sense of the word, involving the whole of Europe and even, towards its end, the United States of America.
"Modern" it became indeed when one considers the new, deadlier weapons that were developed just before and during this war: Tanks, airplanes, machine guns, chemical weapons are the most widely known by the general public.
However, "old fashioned" tactics were used in every day combat tactics. Trench warfare and massive offensives designed by old world general sipping cognac in their comfortable leather armchairs, sending foot soldiers to be butchered to an almost certain death as if they were young boys simulating a battle with lead soldiers, are there to remind us that this war was not "franche et joyeuse" (clearcut: "brave and joyous" is the best translation I could think of).
World War one is indeed the first modern war, as far as the weapons developed and used were concerned, but in my opinion the tactics used in the war were closer to those of the Napoleonic wars than the later 20th century wars.
A modern aspect of the First World War was the weaponry used or developed. During the First World War a wide range of new weapons appeared. Old weapons were rediscovered and redesigned to fulfill "modern" purposes. This is the case of the Flame thrower or the mortars. The use of a flamethrower is already mentioned in the 5th century B.C. Its use in a modern version by the German army brought terror to the British and